This invention relates to optoelectronic semiconductor devices such as a laser diode, a light emitting diode or a photo diode in hermetically sealed packages, and more particularly to such devices having heat sink structure.
One such laser diode is disclosed in U.S. Pat. No. 3,869,702. There, one side of a laser diode is mounted onto a heat sink consisting of a copper block which is in turn secured on the top of a steel stud having a hole therein. The heat sink itself is positioned to cover just less than a semi-cylinder of the stud hole so that its side face is displaced from an optical axis of the laser diode by an amount which will cause a laser diode mounted thereon to lie on axis. A hollow stem protruding from the opposite face of the stud permits passage of a lead through the hollow stem for providing the other connection for the laser diode. The lead is electrically insulated from the hollow stem by means of a sealing glass.
It is well known that the temperature of the laser diode must be maintained as low as possible, while the laser diode should be contained in a hermetically sealed package. Therefore, when a heat sink is made of a high thermal conductivity material such as copper, while a stud is made of a material such as steel a Kovar having a thermal coefficient of expansion close to sealing glass, an excellent hermetically sealed package will be made. (Kovar is a trademark for an alloy containing approximately 29% nickel, 17% cobalt and 0.3% manganese, the balance being essentially iron.) However, for such an excellent package, when either ambient temperature or output power is varied, it is inevitable to cause a thermal stress between the copper block and the steel stud due to the different thermal coefficients of expansion. In the prior art structure, since the bottom of the copper block is positioned on half of the steel stud, the thermal stress is caused only at one side thereof. Such unbalanced partial thermal stress deviates the optical axis of the laser diode from the center axis of the stud. In the past, the above-mentioned axial deviation has never been a concern. In the case of coupling a laser light to a single mode optical fiber, however, axial deviation preferably should be restricted within 0.5 microns. It is hardly possible with conventionally packaged semiconductor devices to achieve such a strict tolerance of the axial deviation.